using System;
using System.Collections.Concurrent;
using System.Net;

namespace AyaNova.Util
{
    /// <summary>
    /// case 4726: failed logins counted per login name and per client address (in memory, a restart resets the counts).
    /// Too many failures within the window and further attempts are refused until the window ends.
    /// Login names that don't exist are counted the same way, so the refusal says nothing about which logins exist.
    /// Requests from this machine (loopback) are not counted per address: behind the hosted nginx and IIS the real
    /// client address is seen, so loopback is only the machine itself (and the integration tests)
    /// </summary>
    internal static class LoginThrottle
    {
        internal const int MAX_LOGIN_FAILURES = 10;
        internal const int MAX_ADDRESS_FAILURES = 50;
        private static readonly TimeSpan WINDOW = TimeSpan.FromMinutes(15);
        private const int PRUNE_ABOVE = 10000;

        private record Window(DateTime Start, int Count);
        private static readonly ConcurrentDictionary<string, Window> Failures = new ConcurrentDictionary<string, Window>();

        private static string LoginKey(string login) => "L:" + (login ?? string.Empty).Trim().ToLowerInvariant();
        private static string AddressKey(IPAddress address) => address == null || IPAddress.IsLoopback(address) ? null : "A:" + address.ToString();

        /// <summary>
        /// True if this login or this address has reached its limit in the current window
        /// </summary>
        internal static bool IsRefused(string login, IPAddress address)
        {
            return Reached(LoginKey(login), MAX_LOGIN_FAILURES) || Reached(AddressKey(address), MAX_ADDRESS_FAILURES);
        }

        /// <summary>
        /// Counts a failed login; returns a description when this failure reached a limit (for the log), otherwise null
        /// </summary>
        internal static string RecordFailure(string login, IPAddress address)
        {
            if (Failures.Count > PRUNE_ABOVE)
                Prune();
            string reached = null;
            if (Count(LoginKey(login)) == MAX_LOGIN_FAILURES)
                reached = $"login \"{login}\"";
            var addressKey = AddressKey(address);
            if (addressKey != null && Count(addressKey) == MAX_ADDRESS_FAILURES)
                reached = (reached == null ? "" : reached + " and ") + $"address {address}";
            return reached;
        }

        /// <summary>
        /// A correct password clears the count for that login
        /// </summary>
        internal static void RecordSuccess(string login)
        {
            Failures.TryRemove(LoginKey(login), out _);
        }

        private static bool Reached(string key, int max)
        {
            return key != null && Failures.TryGetValue(key, out var w) && DateTime.UtcNow - w.Start < WINDOW && w.Count >= max;
        }

        private static int Count(string key)
        {
            var now = DateTime.UtcNow;
            return Failures.AddOrUpdate(key, new Window(now, 1), (_, w) => now - w.Start < WINDOW ? w with { Count = w.Count + 1 } : new Window(now, 1)).Count;
        }

        private static void Prune()
        {
            var now = DateTime.UtcNow;
            foreach (var kv in Failures)
                if (now - kv.Value.Start >= WINDOW)
                    Failures.TryRemove(kv.Key, out _);
        }
    }
}